High frequency modulation of laser beams is an important tool in many fields, such as communication, atomic physics and many others. In particular, the experimental realization of a Λ-system sets the ground for many applications of precision measurements.
Some of these applications include atomic clocks [1, 2], magnetic sensors [3] and gravity gradiometers [4]. This Λ-system consists of two coherent laser fields, two hyperfine levels of an atomic ground state and an excited state. Depending on the application, the coherent coupling of the two sub levels of the ground state is done by laser beams which are resonant (as in CPT) or non resonant (as in Stimulated Raman transitions) with the transition to the excited state. In both cases the beams must be phase locked and with a tunable frequency difference in the range of several GHz (corresponding to the ground state's hyperfine split energy in alkali atoms).
Three main methods have been developed over the years for the generation of the two phase-locked beams: Direct light modulation either by AOM [5] or EOM [6, 7], optical phase locking of two lasers [8, 9] and direct current modulation of a laser diode [10]. The modulation of the DC current injected to the laser diode by an AC signal of the frequency fm produces optical side bands. The first order sidebands can then be injected to slave lasers for amplification and spectral purification. The final result is two phase locked laser beams with a frequency difference of 2fm. However, the modulation response of edge emitting diode laser decreases sharply as the modulation frequency increases.
An alternative is to modulate the current of a vertical cavity surface-emitting laser (VCSEL) [11] which is much more susceptible to high frequency modulation but has very little power (total of 2 mw).
In the present patent application experimental results are presented, demonstrating the modulation response enhancement of an AR coated edge emitting laser diode in an external cavity.
By eliminating the internal cavity of the laser diode and matching the modulation frequency to the FSR of the external cavity per the present invention the modulation index is enhanced to the point of complete carrier suppression even at high modulation frequency. The result is a tunable modulation source in the range of 3 to 6 GHz, with more then 60% of the total power of the output beam concentrated in the two first optical sidebands.
Modulation enhancement by an external resonance cavity with a mode spacing corresponding to the desired modulation frequency was already reported both for edge emitting diodes [12] and VCSEL [13]. However, as the internal resonance cavity of these diodes was not eliminated, complete carrier suppression was not demonstrated in these experiments, or complete carrier suppression was demonstrated, only when the power of the beam was small.
The modulation enhancement of a “regular” (FP) diode (like the one used in [12]) is compared to that of the AR coated diode, to illustrate the effect of the elimination of the internal resonance cavity. The affect of the DC current on the modulation response is also examined.
FIG. 1 of the prior art is a schematic illustration of a resonance cavity 30 of distance L, having back reflector 1 which is a high reflectivity mirror, and an output coupler 2 which is a mirror with partial transmission.
As used herein the specification and in the claims section that follows, the term “FSR”, (free spectral range), and the like refer to the basic resonance frequency of a resonance cavity.
FSR is given by:
                                          F            ⁢                                                  ⁢            S            ⁢                                                  ⁢            R                    =                      c                          2              ⁢                              L                eff                                                    ,                              L            eff                    =                                                    L                1                            ⁢                              n                1                                      +                                          L                2                            ⁢                              n                2                                      +                                          (                                  L                  -                                      L                    1                                    -                                      L                    2                                                  )                            ⁢              n                                                          (        1        )            were c is the speed of light and Leff is the effective length of the resonance cavity.    Leff=L, if the space inside the resonance cavity is empty.    Leff=n×L, if the space inside the resonance cavity is filled with a material with refractive index n.    Leff=n1×L1+n2×L2+ . . . if there are several materials inside the resonance cavity.
FIG. 2 of the prior art is a schematic illustration of a resonance cavity 30 with several materials, having several indexes n along its length L. The first material is spread along L1 and has indexes n=n1, the second material is spread along L2 and has indexes n=n2, etc.
Lasing Frequency
The laser will emit light at a frequency ν that is an integer multiplication of the FSR (resonance condition):ν=k×FSR,k=1,2,3, . . .  (2)
Typically, the value of k for an operating laser is between 105 and 107.
Direct Laser Modulation
In some lasers it is possible to introduce periodic perturbation to the amplification means (gain medium) of the laser. As a result, the laser light's spectrum will include additional components, known as sidebands. In this case the laser field is given by:E=E0ei ωt+Ek±ei(ω±kωm)t,k=1,2,3 . . .  (3)
Where ω=2πν is the main laser angular frequency (also known as the carrier frequency), and ωm=2πfm is the modulation angular frequency. E0 is the amplitude of the carrier and Ek± are the amplitudes of the sidebands.
Modulation Index (Depth)
The amplitudes of the carrier and the sidebands are approximately propositional to the Bessel functions of the first kind:E0∝J0(m),Ek±∝±Jk(m)  (4)
The dimensionless parameter m is an indicator to the modulation strength. For m<0.1 there will be no noticeable modulation. At m=0.4, the first order sidebands (k=1) will have about 4% of the energy each, and the second sideband will emerge. At m=2.4, the carrier amplitude goes to zero (“full carrier suppression”) and all the energy is split between the first 4 sidebands. As m increases, more sidebands will appear in the laser beam.
Typically, direct modulation of a laser beam is achieved by modulating the DC current supply of a laser diode with an AC current source. Changing the AC frequency directly controls the modulation angular frequency ωm. The modulation index m is dependent on several factors: The ratio between the DC and AC power, the dissipation of AC along the diode feeding circuit, reflection of part of the AC power due to imperfect impedance matching and the susceptibility of the diode's material to the AC modulation. On top of all these parameters, as the modulation frequency goes up to the GHz range, the optical sidebands move noticeably away from the resonance condition (2), and are thus strongly suppressed by the laser resonance cavity itself, as shown in FIG. 3.
FIG. 3 of the prior art is a graphical illustration of the suppression of the amplitude of the modulated sidebands due to the effect of the resonance condition
There is a need for a method and a device for enhancing the modulation efficiency of lasers.